drm/i915: Allow a request to be cancelled

If we preempt a request and remove it from the execution queue, we need
to undo its global seqno and restart any waiters.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223074422.4125-11-chris@chris-wilson.co.uk